This invention relates generally to series capacitor installations in high voltage alternating current transmission lines.
Capacitor banks are connected in series in high voltage transmission lines carrying AC power to compensate for the inductance of the line. This compensation is desirable to improve the stability of the system, control load division between parallel lines, and provide other benefits. Since the capacitors are in series with the line, they are subject to overvoltages in case of a fault on the line or other excess current conditions as may result from switching surges. These conditions could result in damage to the capacitors. To avoid such damage, various forms of protective apparatus has been used in association with the series capacitors for the purpose of responding to indications of fault conditions to bypass and effectively remove the series capacitors from the line so they are not subjected to the fault conditions. The capacitors may be reinserted upon clearing of the fault.
A general form of such apparatus is to use a bypass device of substantial current carrying capacity, usually a spark gap or series of spark gaps, connected in a circuit branch across, or in parallel with, the series capacitors. The protective gaps arc over and bypass the capacitors rapidly upon the occurrence of a predetermined fault condition. Apparatus for causing the bypass to occur as well as to provide arc extinction and reinsertion of the capacitors in the line is generally known. Reference is made to the following patents for representative background information with respect to the purpose and utility of such protection systems and their general nature: Grove et al, U.S. Pat. Nos. 3,801,870, Apr. 2, 1974; Ringler et al, 3,816,800, June 11, 1974; and Peterson, 3,889,158, June 10, 1975.
The capacitor protective apparatus is thus normally present primarily for the purpose of protecting against damage to the series capacitors. An AC transmission line system is, however, subject to other conditions that may impose a hazard to parts of the system other than the capacitors. For example, it has been recognized that a generator supplying power to the transmission line is subject to the occurrence of subsynchronous currents in its armature or stator winding, concurrent with the normally present 60 Hz current, resulting in the production of pulsating torques on the generator rotor shaft at the corresponding slip frequency between 60 Hz and the subsynchronous frequency. The subsynchronous frequencies are the result of the use of series capacitor compensation which has a characteristic natural resonant frequency with the line.
This general problem of subsynchronous oscillation is potentially solvable by several distinct techniques including modification of the generator and its exciter or filtering of the undesired slip frequency. Another way is that which the above referred to U.S. Pat. No. 3,889,158 describes. In that patent it is recognized that if the capacitor bank is effectively removed from the system, the undesired subsynchronous oscillation cannot occur. But the voltage level at which sparkover of the capacitor bypass equipment occurs is important because, in general, the higher the sparkover voltage is, the greater the severity of mechanical shock. The concept of that patent is to employ a "dual sparkover" protection system with multiple bypass circuit branches of different sensitivity to overvoltages. One branch is primarily for capacitor protection and its sensitivity is related to the required level of protection for the capacitors themselves. Another branch is more sensitive than that required for capacitor protection and will respond to an overvoltage of lesser magnitude coincident with the occurrence of the subsynchronous oscillation condition. In this way, the severity of mechanical shocks occurring in the generator can, in some applications, be reduced to a tolerable level.
While such techniques are useful and effective, various transmission systems present a variety of specific circumstances and conditions so that the occurrence of subsynchronous oscillations is not uniform on all systems and the effectiveness as well as the overall cost-benefit ratio can vary to an extent that there is a need for alternate approaches and it is to such purpose that the present invention is directed.